The present invention relates to methods applicable to production of semiconductor devices. More specifically, the present invention relates to a variety of methods which can be employed either individually or in combination in the production of wafers of a semiconductor, and particularly of silicon (hereinafter referred to as Si.), to utilize the function of the intrinsic gettering effect on the semiconductor wafers. More precisely, the present invention relates to a variety of methods for production of wafers of a semiconductor, specifically of Si, wherein the semiconductor wafers have a shallow or surface region, which is a denuded zone which is clear and thin enough to enable the intrinsic gettering effect to function to a satisfactory extent, and a deep or internal region which contains a high concentration of crystalline defects having a large size which allow an increase of the intrinsic gettering effect.
An intrinsic gettering effect is effective to produce and maintain a denuded zone which is fairly free from crystalline defects and which is employed as the active layer of a semiconductor device. This effect which is publicly known in the prior art, is based on a principle that an internal region of a semiconductor wafer containing a large quantity or concentration of crystalline defects, has a function to absorb the impurities e.g. oxygen, carbon, et al., each of which has the possibility of producing crystalline defects therewith and which are contained in the surface region of the semiconductor wafer. The remaining surface region of the semiconductor wafer has a defect free portion, that is, the remaining surface region has a thickness range from several micrometers through several tens of micrometers of the semiconductor wafer and acts as a denuded zone.
It is known that a smaller thickness of a denuded zone located in the shallow region of a semiconductor wafer, as well as a larger concentration of crystalline defects contained in the deep or internal region of the semiconductor wafer, is effective to cause a larger intrinsic gettering effect. Therefore, taken only from the viewpoint of this intrinsic gettering effect, a smaller thickness is preferable for a denuded zone. However, since the thickness of a denuded zone is required to exceed that of an active layer of which exemplary thicknesses are 5 micrometers, and 3 micrometers, respectively for a bipolar transistor and for a MOS transistor, the thickness of the denuded zone is required to be approximately 10 micrometers for a bipolar transistor and 6 micrometers for a MOS transistor.
In the prior art, two different methods are available to produce a semiconductor wafer which is allowed to utilize the function of the intrinsic gettering effect. One method produces a semiconductor which has a shallow or surface region comprising a denuded zone free of crystalline defects and a deep or internal region containing crystalline defects large in size. This is a two-step method which comprises a first step in which a semiconductor wafer, containing impurities e.g. oxygen, carbon et al., is annealed at a relatively low temperature (in the range of 600.degree. through 900.degree. C.) for the purpose of producing a high concentration of defect nuclei in the semiconductor wafer, and a second step in which the semiconductor wafer is annealed at a relatively high temperature (in excess of 1,000.degree. C.) for the purpose of causing the high concentration of defect nuclei to grow to a high concentration of crystalline defects. The other method is a three-step method which is an improvement of the foregoing two-step method. The three-step method comprises (a) a first step in which a semiconductor wafer produced employing the Czochralski method and which contains impurities, e.g. oxygen with a concentration range of 0.5 through 2.0.times.10.sup.18 /cm.sup.3, is annealed for approximately 20 hours in nitrogen gas at a high temperature of approximately 1,100.degree. C. for the purpose of diffusing the impurities out of the shallow region or the surface region of the semiconductor wafer; (b) a second step in which the semiconductor wafer is annealed for approximately 40 hours in nitrogen gas at a relatively low temperature, in the approximate temperature range of 600.degree. through 900.degree. C., for the purpose of causing the impurities remaining in the deep region or the internal region of the semiconductor wafer to grow to a high concentration of defect nuclei, the remaining shallow region or the surface region of the semiconductor wafer contain no impurities, that is a defect-free region, and (c) a third step in which the semiconductor wafer is annealed for approximately 20 hours in nitrogen gas at a relatively high temperature of approximately 1,050.degree. C. for the purposes (i) of causing the high concentration of defect nuclei to grow to a high concentration of crystalline defects in the deep region or the internal region of the semiconductor wafer and (ii) of producing a denuded zone in the shallow region or the surface region.
Any of the foregoing methods available in the prior art for producing a semiconductor wafer which is allowed to utilize the function of the intrinsic gettering effect has drawbacks described below. A first drawback is the extremely long period of time required for the process. Specifically, in the case of the three-step method, the process requires approximately 80 hours, which is too long particularly from a realistic viewpoint. Albeit the two-step method requires a shorter length of time, but the quality of the denuded zone produced employing this method is unsatisfactory in comparison with the quality of the denuded zone produced employing the three-step method. A second drawback is caused by an unsatisfactory thickness of a protection layer which covers the surface of a semiconductor wafer during the period in which one of the foregoing methods of production of the semiconductor wafer which utilizes the function of the intrinsic gettering effect for the purpose of inhibiting surface defects from appearing, particularly in the surface region of the semiconductor wafer. This unsatisfactory thickness of the surface protection results in a requirement of an additional step to remove the surface region containing the foregoing surface defects. The third drawback is caused by unsatisfactory control of the thickness of the denuded zone. In other words, it is not necessarily easy to control the thickness of the denuded zone produced employing any of the foregoing methods, particularly because this control is influenced by the grade of concentration of oxygen contained in a semiconductor wafer. A fourth drawback is a possibility in which a semiconductor wafer warps during the process, due to a rather high temperature required. A fifth drawback is a possibility in which a denuded zone, once produced in a semiconductor wafer, is rather easily lost during later thermal processes which inevitably are contained in a process for producing a semiconductor device. A sixth drawback is the rather unsatisfactory magnitude of the intrinsic gettering effect which can be expected from any of the semiconductor wafers produced employing any of the methods based on annealing processes, particularly in the case where a bipolar semiconductor is produced in such a semiconductor wafer.